1. Field of the Invention
This invention generally relates to the field of power converters. More specifically, the invention relates to the field of reducing noise in power converters such as AC to DC converters, DC to AC converters or DC to DC converters.
2. Background of the Invention
A modern switching-mode power converter is light weight and provides high efficiency power conversion. One of the problems with this type of converter, however, is that it generates undesirable switching noise. This switching noise manifests itself as a ripple voltage or ripple current that is generated by the switching-mode at the input or output side of the converter. Reduction of such ripple voltage or current becomes a necessary design requirement in order to comply with international standards. Some prior art provides a means to reduce such ripple current. For example, a traditional means to suppress ripple voltage or current is by implementing a passive filter. As shown in FIG. 1, a prior art converter implements a simple LC circuit. A pair of inductors, L1 and L2 are coupled to a pair of capacitors C1 and C2. Together, these components are coupled to a converter that is modeled as a diode D1, a capacitor C3, a resistor R1 and a transistor M1 that acts as a switch for the converter. A passive filter, however, requires bulky components that take up the limited space within the power converter. Other prior art work implements active cancellation techniques to reduce the ripple noise. For example, in U.S. Pat. No. 4,274,133 to Cuk a converter is disclosed that cancels ripple current by matching a coupling coefficient of two inductors within the converter. In U.S. Pat. No. 5,038,263 to Marrero a circuit is disclosed with a winding coupled to the main transformer of the converter for diverting ripple current to a capacitor. The coupling ratio of the transformer windings reduces the ac ripple current input switching. In Marrero, an inductor is coupled in series with the input to provide constant current. This inductor, however, is bulky and adds an extra component in the power conversion path. In U.S. Pat No. 6,008,999 also to Marrero a converter is disclosed having an additional winding that effects the output inductance of the converter. The coupling ratio of the additional winding and output winding reduces input and output switching ripple.
These prior art methods have drawbacks that limit the utility of the power converter. For example, traditional passive filters require bulky inductors and capacitors that increase component count and space requirements. Known active noise cancellation techniques reduce component size and count, but require careful magnetic coupling between each winding in the main magnetic component. The cancellation effect is not achieved in these circuits without a tightly-coupled magnetic field. The main magnetic component must be made precisely to satisfy both power conversion and noise cancellation requirements. This becomes an added constraint to the design and increases the difficulty to manufacture the converter.
The present invention provides a general solution for canceling ripple current generated by a power converter. In general, two methods are provided, a series voltage source or a shunt current source, which are placed at the input terminals of a power converter to eliminate ripple current generated by the converter. This noise cancellation apparatus can be applied to any power converter because it is a separate unit that captures the noise signal and generates a cancellation signal equal in magnitude, but opposite in phase to the noise signal, in order to reduce the undesirable noise, such as switching voltage or ripple current of a switching-mode power supply.
According to one aspect of the invention, an AC voltage can be inserted in series with the input terminal of the power converter to reduce the noise voltage. The AC voltage is equal in magnitude but opposite in phase to the noise voltage generated by an AC noise current, or the equivalent of an AC noise current, coupled to the input impedance of the power converter. The noise voltage at the input is thus cancelled.
According to another aspect of the invention, a shunt AC current source can be inserted in parallel with the input terminal of a power converter. If the shunt AC current has equal magnitude, but opposite phase, of the noise current generated by the power converter, then the AC current flow into the input terminal is cancelled.
According to yet another aspect of the invention, an AC voltage can be inserted in series with the input terminal of a power converter. This voltage source can be derived from a noisy voltage node or its equivalent within the converter. Voltage scaling is provided by impedance networks and this voltage source can cancel out noise at the converter input.
The design of these cancellation circuits, unlike traditional filter designs, is not dependent on converter input source impedance. The noise cancellation can also be incorporated with a traditional filter to further reduce the noise level associated with the ripple current and voltage.
Accordingly, it is an object of the present invention to provide noise cancellation circuits for power converters. The noise cancellation circuits can be placed on the input terminals or output terminals of the power converter. The noise reduction apparatus can reduce the noise of different topologies of power converters. Furthermore, the noise reduction apparatus has a low component count and reduces the size of the components in the design.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and from the accompanying drawings.